1 //===- ObjCARCAnalysisUtils.h - ObjC ARC Analysis Utilities -----*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 /// \file 9 /// This file defines common analysis utilities used by the ObjC ARC Optimizer. 10 /// ARC stands for Automatic Reference Counting and is a system for managing 11 /// reference counts for objects in Objective C. 12 /// 13 /// WARNING: This file knows about certain library functions. It recognizes them 14 /// by name, and hardwires knowledge of their semantics. 15 /// 16 /// WARNING: This file knows about how certain Objective-C library functions are 17 /// used. Naive LLVM IR transformations which would otherwise be 18 /// behavior-preserving may break these assumptions. 19 /// 20 //===----------------------------------------------------------------------===// 21 22 #ifndef LLVM_LIB_ANALYSIS_OBJCARCANALYSISUTILS_H 23 #define LLVM_LIB_ANALYSIS_OBJCARCANALYSISUTILS_H 24 25 #include "llvm/ADT/Optional.h" 26 #include "llvm/Analysis/AliasAnalysis.h" 27 #include "llvm/Analysis/ObjCARCInstKind.h" 28 #include "llvm/Analysis/ValueTracking.h" 29 #include "llvm/IR/Constants.h" 30 #include "llvm/IR/Module.h" 31 #include "llvm/IR/ValueHandle.h" 32 33 namespace llvm { 34 namespace objcarc { 35 36 /// A handy option to enable/disable all ARC Optimizations. 37 extern bool EnableARCOpts; 38 39 /// Test if the given module looks interesting to run ARC optimization 40 /// on. 41 inline bool ModuleHasARC(const Module &M) { 42 return 43 M.getNamedValue("llvm.objc.retain") || 44 M.getNamedValue("llvm.objc.release") || 45 M.getNamedValue("llvm.objc.autorelease") || 46 M.getNamedValue("llvm.objc.retainAutoreleasedReturnValue") || 47 M.getNamedValue("llvm.objc.unsafeClaimAutoreleasedReturnValue") || 48 M.getNamedValue("llvm.objc.retainBlock") || 49 M.getNamedValue("llvm.objc.autoreleaseReturnValue") || 50 M.getNamedValue("llvm.objc.autoreleasePoolPush") || 51 M.getNamedValue("llvm.objc.loadWeakRetained") || 52 M.getNamedValue("llvm.objc.loadWeak") || 53 M.getNamedValue("llvm.objc.destroyWeak") || 54 M.getNamedValue("llvm.objc.storeWeak") || 55 M.getNamedValue("llvm.objc.initWeak") || 56 M.getNamedValue("llvm.objc.moveWeak") || 57 M.getNamedValue("llvm.objc.copyWeak") || 58 M.getNamedValue("llvm.objc.retainedObject") || 59 M.getNamedValue("llvm.objc.unretainedObject") || 60 M.getNamedValue("llvm.objc.unretainedPointer") || 61 M.getNamedValue("llvm.objc.clang.arc.use"); 62 } 63 64 /// This is a wrapper around getUnderlyingObject which also knows how to 65 /// look through objc_retain and objc_autorelease calls, which we know to return 66 /// their argument verbatim. 67 inline const Value *GetUnderlyingObjCPtr(const Value *V, 68 const DataLayout &DL) { 69 for (;;) { 70 V = GetUnderlyingObject(V, DL); 71 if (!IsForwarding(GetBasicARCInstKind(V))) 72 break; 73 V = cast<CallInst>(V)->getArgOperand(0); 74 } 75 76 return V; 77 } 78 79 /// A wrapper for GetUnderlyingObjCPtr used for results memoization. 80 inline const Value * 81 GetUnderlyingObjCPtrCached(const Value *V, const DataLayout &DL, 82 DenseMap<const Value *, WeakTrackingVH> &Cache) { 83 if (auto InCache = Cache.lookup(V)) 84 return InCache; 85 86 const Value *Computed = GetUnderlyingObjCPtr(V, DL); 87 Cache[V] = const_cast<Value *>(Computed); 88 return Computed; 89 } 90 91 /// The RCIdentity root of a value \p V is a dominating value U for which 92 /// retaining or releasing U is equivalent to retaining or releasing V. In other 93 /// words, ARC operations on \p V are equivalent to ARC operations on \p U. 94 /// 95 /// We use this in the ARC optimizer to make it easier to match up ARC 96 /// operations by always mapping ARC operations to RCIdentityRoots instead of 97 /// pointers themselves. 98 /// 99 /// The two ways that we see RCIdentical values in ObjC are via: 100 /// 101 /// 1. PointerCasts 102 /// 2. Forwarding Calls that return their argument verbatim. 103 /// 104 /// Thus this function strips off pointer casts and forwarding calls. *NOTE* 105 /// This implies that two RCIdentical values must alias. 106 inline const Value *GetRCIdentityRoot(const Value *V) { 107 for (;;) { 108 V = V->stripPointerCasts(); 109 if (!IsForwarding(GetBasicARCInstKind(V))) 110 break; 111 V = cast<CallInst>(V)->getArgOperand(0); 112 } 113 return V; 114 } 115 116 /// Helper which calls const Value *GetRCIdentityRoot(const Value *V) and just 117 /// casts away the const of the result. For documentation about what an 118 /// RCIdentityRoot (and by extension GetRCIdentityRoot is) look at that 119 /// function. 120 inline Value *GetRCIdentityRoot(Value *V) { 121 return const_cast<Value *>(GetRCIdentityRoot((const Value *)V)); 122 } 123 124 /// Assuming the given instruction is one of the special calls such as 125 /// objc_retain or objc_release, return the RCIdentity root of the argument of 126 /// the call. 127 inline Value *GetArgRCIdentityRoot(Value *Inst) { 128 return GetRCIdentityRoot(cast<CallInst>(Inst)->getArgOperand(0)); 129 } 130 131 inline bool IsNullOrUndef(const Value *V) { 132 return isa<ConstantPointerNull>(V) || isa<UndefValue>(V); 133 } 134 135 inline bool IsNoopInstruction(const Instruction *I) { 136 return isa<BitCastInst>(I) || 137 (isa<GetElementPtrInst>(I) && 138 cast<GetElementPtrInst>(I)->hasAllZeroIndices()); 139 } 140 141 /// Test whether the given value is possible a retainable object pointer. 142 inline bool IsPotentialRetainableObjPtr(const Value *Op) { 143 // Pointers to static or stack storage are not valid retainable object 144 // pointers. 145 if (isa<Constant>(Op) || isa<AllocaInst>(Op)) 146 return false; 147 // Special arguments can not be a valid retainable object pointer. 148 if (const Argument *Arg = dyn_cast<Argument>(Op)) 149 if (Arg->hasPassPointeeByValueAttr() || Arg->hasNestAttr() || 150 Arg->hasStructRetAttr()) 151 return false; 152 // Only consider values with pointer types. 153 // 154 // It seemes intuitive to exclude function pointer types as well, since 155 // functions are never retainable object pointers, however clang occasionally 156 // bitcasts retainable object pointers to function-pointer type temporarily. 157 PointerType *Ty = dyn_cast<PointerType>(Op->getType()); 158 if (!Ty) 159 return false; 160 // Conservatively assume anything else is a potential retainable object 161 // pointer. 162 return true; 163 } 164 165 inline bool IsPotentialRetainableObjPtr(const Value *Op, 166 AliasAnalysis &AA) { 167 // First make the rudimentary check. 168 if (!IsPotentialRetainableObjPtr(Op)) 169 return false; 170 171 // Objects in constant memory are not reference-counted. 172 if (AA.pointsToConstantMemory(Op)) 173 return false; 174 175 // Pointers in constant memory are not pointing to reference-counted objects. 176 if (const LoadInst *LI = dyn_cast<LoadInst>(Op)) 177 if (AA.pointsToConstantMemory(LI->getPointerOperand())) 178 return false; 179 180 // Otherwise assume the worst. 181 return true; 182 } 183 184 /// Helper for GetARCInstKind. Determines what kind of construct CS 185 /// is. 186 inline ARCInstKind GetCallSiteClass(const CallBase &CB) { 187 for (auto I = CB.arg_begin(), E = CB.arg_end(); I != E; ++I) 188 if (IsPotentialRetainableObjPtr(*I)) 189 return CB.onlyReadsMemory() ? ARCInstKind::User : ARCInstKind::CallOrUser; 190 191 return CB.onlyReadsMemory() ? ARCInstKind::None : ARCInstKind::Call; 192 } 193 194 /// Return true if this value refers to a distinct and identifiable 195 /// object. 196 /// 197 /// This is similar to AliasAnalysis's isIdentifiedObject, except that it uses 198 /// special knowledge of ObjC conventions. 199 inline bool IsObjCIdentifiedObject(const Value *V) { 200 // Assume that call results and arguments have their own "provenance". 201 // Constants (including GlobalVariables) and Allocas are never 202 // reference-counted. 203 if (isa<CallInst>(V) || isa<InvokeInst>(V) || 204 isa<Argument>(V) || isa<Constant>(V) || 205 isa<AllocaInst>(V)) 206 return true; 207 208 if (const LoadInst *LI = dyn_cast<LoadInst>(V)) { 209 const Value *Pointer = 210 GetRCIdentityRoot(LI->getPointerOperand()); 211 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Pointer)) { 212 // A constant pointer can't be pointing to an object on the heap. It may 213 // be reference-counted, but it won't be deleted. 214 if (GV->isConstant()) 215 return true; 216 StringRef Name = GV->getName(); 217 // These special variables are known to hold values which are not 218 // reference-counted pointers. 219 if (Name.startswith("\01l_objc_msgSend_fixup_")) 220 return true; 221 222 StringRef Section = GV->getSection(); 223 if (Section.find("__message_refs") != StringRef::npos || 224 Section.find("__objc_classrefs") != StringRef::npos || 225 Section.find("__objc_superrefs") != StringRef::npos || 226 Section.find("__objc_methname") != StringRef::npos || 227 Section.find("__cstring") != StringRef::npos) 228 return true; 229 } 230 } 231 232 return false; 233 } 234 235 enum class ARCMDKindID { 236 ImpreciseRelease, 237 CopyOnEscape, 238 NoObjCARCExceptions, 239 }; 240 241 /// A cache of MDKinds used by various ARC optimizations. 242 class ARCMDKindCache { 243 Module *M; 244 245 /// The Metadata Kind for clang.imprecise_release metadata. 246 llvm::Optional<unsigned> ImpreciseReleaseMDKind; 247 248 /// The Metadata Kind for clang.arc.copy_on_escape metadata. 249 llvm::Optional<unsigned> CopyOnEscapeMDKind; 250 251 /// The Metadata Kind for clang.arc.no_objc_arc_exceptions metadata. 252 llvm::Optional<unsigned> NoObjCARCExceptionsMDKind; 253 254 public: 255 void init(Module *Mod) { 256 M = Mod; 257 ImpreciseReleaseMDKind = NoneType::None; 258 CopyOnEscapeMDKind = NoneType::None; 259 NoObjCARCExceptionsMDKind = NoneType::None; 260 } 261 262 unsigned get(ARCMDKindID ID) { 263 switch (ID) { 264 case ARCMDKindID::ImpreciseRelease: 265 if (!ImpreciseReleaseMDKind) 266 ImpreciseReleaseMDKind = 267 M->getContext().getMDKindID("clang.imprecise_release"); 268 return *ImpreciseReleaseMDKind; 269 case ARCMDKindID::CopyOnEscape: 270 if (!CopyOnEscapeMDKind) 271 CopyOnEscapeMDKind = 272 M->getContext().getMDKindID("clang.arc.copy_on_escape"); 273 return *CopyOnEscapeMDKind; 274 case ARCMDKindID::NoObjCARCExceptions: 275 if (!NoObjCARCExceptionsMDKind) 276 NoObjCARCExceptionsMDKind = 277 M->getContext().getMDKindID("clang.arc.no_objc_arc_exceptions"); 278 return *NoObjCARCExceptionsMDKind; 279 } 280 llvm_unreachable("Covered switch isn't covered?!"); 281 } 282 }; 283 284 } // end namespace objcarc 285 } // end namespace llvm 286 287 #endif 288